1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display (LCD) device and a method of driving the same.
2. Discussion of the Related Art
Until recently, display devices have typically used cathode-ray tubes (CRTs). Presently, many efforts and studies are being made to develop various types of flat panel displays, such as liquid crystal display (LCD) devices, plasma display panels (PDPs), field emission displays, and electro-luminescence displays (ELDs), as a substitute for CRTs. Of these flat panel displays, LCD devices have many advantages, such as high resolution, light weight, thin profile, compact size, and low voltage power supply requirements.
In general, an LCD device includes two substrates that are spaced apart and face each other with a liquid crystal material interposed between the two substrates. The two substrates include electrodes that face each other such that a voltage applied between the electrodes induces an electric field across the liquid crystal material. Alignment of the liquid crystal molecules in the liquid crystal material changes in accordance with the intensity of the induced electric field into the direction of the induced electric field, thereby changing the light transmissivity of the LCD device. Thus, the LCD device displays images by varying the intensity of the induced electric field.
FIG. 1 is a circuit diagram illustrating a sub-pixel of an LCD device according to the related art.
Referring to FIG. 1, the LCD device includes a gate line GL, a data line DL, a thin film transistor T, a storage capacitor Cst and a liquid crystal capacitor Clc.
The gate and data lines GL and DL crosses each other to define a sub-pixel P, the thin film transistor T is connected to the gate and data lines GL and DL, and the storage capacitor Cst and the liquid crystal capacitor Clc are connected to the thin film transistor T.
Although not shown in the drawings, the liquid crystal capacitor Clc includes a pixel electrode connected to the thin film transistor T, a liquid crystal layer, and a common electrode, and functions to display a gray level corresponding to a data signal applied to the pixel electrode. The storage capacitor Cst stores the data signal for a frame and functions to maintain a pixel voltage Vp of the pixel electrode.
When the thin film transistor T is turned on by a gate signal supplied to the gate line GL, the data signal supplied to the data line DL is applied to the pixel electrode as the pixel voltage Vp. In other words, one electrodes of the liquid crystal capacitor Clc and the storage capacitor Cst are connected to a drain electrode of the thin film transistor T and supplied with the pixel voltage Vp corresponding to the data signal, and other electrodes of the liquid crystal capacitor Clc and the storage capacitor Cst are connected to the common electrode and supplied with a common voltage Vcom.
When the LCD device are operated for a long time, because of the same electric field induced for a long time, optical property of the liquid crystal layer is degraded, or positive or negative charges are accumulated at the liquid crystal layer near the pixel electrode and the common electrode thus the liquid crystal capacitor Clc is deteriorated and display quality degradation such as residual images is caused.
To solve the above problems, proposed is an inversion driving method of alternating polarities of a data signal by the predetermined period and preventing the charge accumulation in the liquid crystal layer.
The inversion driving method is categorized into a dot inversion method, a horizontal line inversion method, a vertical line inversion method, a frame inversion method and the like. The dot inversion method, the horizontal line inversion method, the vertical line inversion method can be used in combination with the frame inversion method.
The dot inversion method is to invert a polarity of a data signal per sub-pixel and per frame thus has the advantage of displaying images having good quality. The dot inversion method is categorized into a 1(one)-dot inversion method, a vertical 2(two)-dot inversion method, a horizontal 2(two)-dot inversion method and the like.
The horizontal line inversion method is to invert a polarity of a data signal per horizontal line and per frame. The vertical line inversion method is to invert a polarity of a data signal per vertical line and per frame.
The frame inversion method is to invert a polarity of a data signal per frame.
When displaying normal images, image display of good quality can be performed by driving the LCD device in the dot inversion method. However, when displaying an image having a specific pattern, for example, an image having different grays arranged in stripe form, display quality degradation such as crosstalk, greenish and the like may occur.
FIG. 2 is a view illustrating a specific pattern image displayed in the LCD device according to the related art.
Referring to FIG. 2, red (R), green (G) and blue (B) sub-pixels are alternately arranged in each horizontal line, and the same color sub-pixels are arranged in each vertical line. This type LCD device may be referred to as a stripe type LCD device. The neighboring red (R), green (G) and blue (B) sub-pixels form a pixel as an image display unit.
The LCD device displays the specific pattern image, in which different grays, for example, black and white are alternately arranged in stripe form, in a dot inversion method. In this case, for a mth horizontal line HLm, a red (R1) data signal for a high gray (white) of a positive polarity (+), a green (G1) data signal for a high gray (white) of a negative polarity (−), a blue (B1) data signal for a high gray (white) of a positive polarity (+), a red (R2) data signal for a low gray (black) of a negative polarity (−), a green (G2) data signal for a low gray (black) of a positive polarity (+), a blue (B2) data signal for a low gray (black) of a negative polarity (−), and the like are inputted to the irrespectively sub-pixels. For a (m+1)th horizontal line HLm+1, a red (R1) data signal for a high gray (white) of a negative polarity (−), a green (G1) data signal for a high gray (white) of a positive polarity (+), a blue (B1) data signal for a high gray (white) of a negative polarity (−), a red (R2) data signal for a low gray (black) of a positive polarity (+), a green (G2) data signal for a low gray (black) of a negative polarity (−), a blue (B2) data signal for a low gray (black) of a positive polarity (+), and the like are inputted to the irrespectively sub-pixels.
As described above, for the mth horizontal line HLm, the data signals having a negative polarity (−) and the data signals having a positive polarity (+) are the same in number. However, the data signals of a positive polarity (+) are dominant in the high gray region displaying white while the data signals of a negative polarity (−) are dominant in the low gray region displaying black, and a voltage of the data signal for white has an absolute value more than a voltage of the data signal for black. Accordingly, the data signals of the mth horizontal line HLm have a positive polarity (+) overall.
On the contrary, for the (m+1)th horizontal line HLm+1, the data signals having a negative polarity (−) and the data signals having a positive polarity (+) are the same in number. However, the data signals of a negative polarity (−) are dominant in the high gray region displaying white while the data signals of a positive polarity (+) are dominant in the low gray region displaying black, and a voltage of the data signal for white has an absolute value more than a voltage of the data signal for black. Accordingly, the data signals in the mth horizontal line HLm+1 have a negative polarity (+) overall.
The data signal is applied to the pixel electrode as a pixel voltage, and the pixel voltage induces an electric field along with a common voltage applied to the common electrode facing the pixel electrode. According to the dominant polarity of the pixel voltages, the common voltage is shifted.
In other words, the common voltage of the mth horizontal line HLm is shifted to have a positive polarity (+) while the common voltage of the (m+1)th horizontal line HLm+1 is shifted to have a negative polarity (−).
Accordingly, with respect to the positively-shifted common voltage of the mth horizontal line HLm, a voltage difference between the green (G) data signal for the high gray (white) of a negative polarity (−) of the mth horizontal line HLm and the common voltage is greater than a voltage difference between each of the red (R) and blue (B) data signals for the high gray (white) of a positive polarity (+) and the common voltage. On the contrary, with respect to the negatively-shifted common voltage of the mth horizontal line HLm+1, a voltage difference between the green (G) data signal for the high gray (white) of a positive polarity (+) of the (m+1)th horizontal line HLm+1 and the common voltage is greater than a voltage difference between each of the red (R) or blue (B) data signal for the high gray (white) of a negative polarity (−) and the common voltage. Accordingly, the green (G) data signal for the high gray level (white) displays a gray level higher than each of the red (R) and blue (B) data signal for the high gray level (white) over the whole of the LCD device.
As described above, when the LCD device operated in a dot inversion method displays the specific pattern image, in which the different grays are alternately arranged in stripe form, the green (G) data signal has the higher gray level and the display image is greenish. Accordingly, display quality is degraded.
Further, when another specific pattern image, in which a rectangular region at center of the image and a peripheral region surrounding the rectangular region are different in gray level and different grays are arranged in stripe form in the rectangular region, is displayed, there occurs a crosstalk that an specific image in stripe form is dimly seen at a portion of the peripheral region that extends horizontally from the rectangular region. Accordingly, display quality is degraded.